Visbreaking, or viscosity breaking, is a well known petroleum refining process in which heavy oils including residual fractions or reduced crudes are pyrolyzed, or cracked, under comparatively mild conditions to provide products having lower viscosities, thus reducing the amounts of less viscous and more valuable blending oils required to make the residual stocks useful as fuel oils. The visbreaker feedstock usually consists of one or more refinery streams derived from sources such as atmospheric residuum, vacuum residuum, furfural extract, propane-deasphalted tar and catalytic cracker bottoms. Most of these feedstock components, except the heavy aromatic oils, behave independently in the visbreaking operation. Consequently, the severity of the operation for a mixed feed is limited greatly by the least desirable (highest coke-forming) components. In a typical visbreaking process, the crude or resid feed is passed through a heater and heated to about 425.degree. to about 525.degree. C. at about 450 to about 7000 kPa. Light gas-oil may be recycled from the product fractionator to quench the visbreaker reactor effluent to about 260.degree. to about 370.degree. C. Cracked products from the reaction are flash distilled with the vapor overhead being fractionated into a light distillate overhead product, for example gasoline and light gas-oil bottoms, and the liquid bottoms are vacuum fractionated into heavy gas-oil distillate and residual tar. Examples of such visbreaking methods are described in Beuther et al, "Thermal Visbreaking of Heavy Residues," The Oil and Gas Journal, 57:46, Nov. 9, 1959, pp. 151-157; Rhoe et al, "Visbreaking: A Flexible Process," Hydrocarbon Processing, January 1979, pp. 131-136; and U.S. Pat. No. 4,233,138.
Various visbreaking processes have been proposed in which residual oils are added at the visbreaking stage with or without added hydrogen or hydrogen-donors. For example, U.S. Pat. No. 3,691,058 describes the production of single ring aromatic hydrocarbons (70.degree.-220.degree. C.) by hydrocracking a heavy hydrocarbon feed (565.degree. C.-) and recycling 32.degree.-70.degree. C. and 220.degree. C.+ product fractions to extinction. This is integrated with visbreaking of residua in the presence of 1 to 28 weight % free radical acceptor at 370.degree.-480.degree. C. in the presence or absence of hydrogen (to enhance residua depolymerization). U.S. Pat. No. 4,067,757 describes a process which involves passing a resid at temperatures from 400.degree.-540.degree. C. up through a bed of inert solids (packed bed reactor) either in the absence of hydrogen or the presence of specific quantities of hydrogen relative to the resid to enhance production of middle distillate. U.S. Pat. No. 4,363,716 discloses an upgrading process for heavy oils using a hydrogen donor solvent which is subjected to hydroprocessing in a recycle sequence. The solvent is used in an amount of at least 25 percent with respect to the feedstock.
U.S. Pat. No. 2,953,513 discloses a hydrogen donor diluent cracking process using hydrogen-donors produced by the partial hydrogenation of certain distillate thermal and catalytic tars, boiling above 370.degree. C., containing a minimum of 40 weight % aromatics. The donors have hydrogen:carbon ratios of 0.7-1.6. The resid feed is mixed with 9-83 volume % of the hydrogen donor and thermally cracked at 427.degree.-482.degree. C. to produce low boiling products. U.S. Pat. No. 4,090,947 describes a thermal cracking process (425.degree.-540.degree. C.) for converting resids to lighter products in the presence of 10-500 volume percent of a hydrogen-donor produced by hydrotreating coker gas oil (345.degree.-480.degree. C.) alone or blended with gas oil produced in the thermal cracker.
A number of proposals have been made for upgrading resids by thermal cracking processes which are operated with hydrogen, either alone or with hydrogen transfer solvents. For example, U.S. Pat. No. 4,292,168 discloses the upgrading of heavy hydrocarbon oils without substantial formation of char by heating the oil with hydrogen and a hydrogen transfer solvent without a catalyst at temperatures of about 320.degree. to 500.degree. C. and at elevated pressure for a time of about 3 to 30 minutes; examples of hydrogen-donor transfer solvents used in this process include pyrene, fluoranthene, anthracene and benzanthracene. U.S. Pat. No. 4,389,303 discloses a process for converting high boiling crudes with high resid contents by carrying out visbreaking, e.g. at temperatures from 380.degree. to 480.degree. C. in the presence of hydrogen as well as a donor solvent. These processes which use free hydrogen have the disadvantage, however, of being relatively expensive both in capital outlay (since pressure vessels and enlarged gas plants are necessary) and operating costs (since hydrogen is expensive). This is so, regardless of whether the hydrogen is contacted directly with the feedstock or used to regenerate the donor solvent. It would therefore be desirable to upgrade resids and other heavy oils without the need for free hydrogen.
Other processes for thermally upgrading heavy oils have been described, for example, in U.S. Pat. Nos. 4,428,824 and 4,389,302. The process described in U.S. Pat. No. 4,428,824 subjects a deasphalted oil to visbreaking, and subsequently reblends it with the asphaltic fraction to produce a product of low viscosity and pour point. This process reduces the amount of cutter stock required to make relatively low viscosity products. The reduction in cutter stock requirement is achieved by minimizing coke formation in the visbreaker, by excluding the worst coke formers, permitting more severe operation of the visbreaker. The process described in U.S. Pat. No. 4,389,302 subjects a heavy oil to sequential separation, visbreaking and separation steps, using an aromatic diluent such as catalytic cracker recycle oil in the visbreaking step. This diluent is used, however, merely to increase the fluidity of the heavy oil which is relatively high in viscosity after the initial separation step in which the lighter components are removed.
These processes have other disadvantages such as the need for solvent deasphalting or separation units but, in particular, they fail to achieve one necessary objective in any heavy oil upgrading step, which is to increase the hydrogen:carbon ratio of the oil without doing this wastefully by simply rejecting carbon as a more refractory resid. Obviously, if hydrogen can be added to the feed without the disadvantages of using molecular hydrogen, a much more economical upgrading process will be at hand.